1. Field of the Invention
The present invention relates to power factor correction circuits and specifically to low cost implementations of boost preregulator circuits with high power factor, low total harmonic distortion and fixed frequency operation.
2. Description of the Prior Art
Power factor correction places the voltage sinusoid and current sinusoid load on an alternating current (AC) power line in phase. When the two are in phase, the power factor is said to be high, approaching unity. High power factor operation of electrical appliances is desirable, and in some situations, mandatory.
A typical uncorrected power supply exhibits a power factor of about 0.65. Therefore, only sixty five percent of the power that can be drawn from a power line will contribute to real power. For a fifteen amp line service at a nominal 115 volts, this translates to 897 watts of real power. A power supply with an eighty percent operating efficiency connected to a 115 volt AC line can supply no more than 717 watts of real power to a system.
Power factor correction circuits can raise a power supply's power factor to near unity, and thereby provide more real power in a limited environment, e.g., the fifteen amp branch circuit common in residential and commercial buildings. At unity, the current waveform is sinusoidal and nearly all the current drawn from the AC line contributes to real power. For an end user, this may make the difference between using a facility's existing wiring or installing a new service, a twenty or thirty amp branch.
An active power factor corrector must control both the input current and output voltage. Ideally, the input to a converter will appear resistive, such as where the current loop is programmed by the rectified line voltage. The output voltage is controlled by changing the average amplitude of the current programming signal. An analog multiplier, such as is typical in the prior art, can be used to create the current programming signal by multiplying the rectified line voltage with the output of a voltage error amplifier. The current programming signal has the shape of the input voltage and an average amplitude is used to control the output voltage.
A squarer and a divider can be combined with a multiplier in the voltage loop of an active power factor corrector control circuit. The output of the voltage error amplifier is divided by the square of the average input voltage before it is multiplied by a rectified input voltage signal. These extra circuits keep the gain of the voltage loop constant, otherwise, the gain of the voltage loop would change as the square of the average input voltage.